1. Field of the Invention
The present invention relates to metrology and more particularly to encoder-based measurement technology. An array of detectors is configured onto a monolithic substrate, which, when illuminated by an interference fringe pattern such as may be derived from a linear encoder, produces an electrical signal by which a determination of the phase of the pattern may be calculated with high precision and resolution. The elements of the detector array are electrically connected in such a way as to produce a number of parallel channels, so that the electrical signals produced by each such channel are identical to each other except for the phase of each channel.
2. Description Relative to the Prior Art
Interferometer techniques for the precise measurements of displacements are well known. Such techniques include, inter-alia, the use of encoders or diffraction gratings attached to the objects whose displacements are to measured, and involve illumination of the gratings by light sources via suitable optics.
Other implementations forego the use of gratings, or scales, and use the surface characteristics of the object to be measured instead.
All such devices require a detector of some type, which transforms the optical signal into an electrical signal, which can then be processed to produce the desired displacement.
Many of the detectors in the past have been photovoltaic devices which produced electrical signals which varied in time in a periodic manner, wherein the number of cycles of the signal was indicative of the motion to be measured.
The so called Moire technique devices have constituted a major type of the prior art in this regard. This technique is well known in the art, and therefore will not be further described herein. U.S. patents utilizing the Moire technique include the following:
______________________________________ U.S. Pat. No. Applicant Date Issued ______________________________________ 3,166,624 Vargady, Leslie O. 01/19/65 3,833,303 Burns et al. 09/03/34 3,910,703 Burns et al. 10/07/75 4,775,788 Harshberger et al. 10/04/88 4,988,886 Greivenkamp et al. 01/29/91 ______________________________________
Other, more recent inventions, utilize an optical signal having a spatial variation, rather than (or as well as) a time variation. The measurement of such a spatial variation requires a structured detector, array of detectors, or the like, in conjunction of the movement of the signal across the structured detector.
U.S. Pat. No. 5,113,386, issued on May 12, 1992, to Marchant, et al., utilizes a four-element array of detectors, with a spatial filter to eliminate the primarily zeroth order region of the detector image.
U.S. Pat. No. 5,098,190, issued on Mar. 24, 1992, to Wintjes et al, and assigned to Optra, Inc., the assignee of the current application, and which is incorporated herein by reference, describes a novel technique for the measurement of scale displacements with three output signals. This technique was also described in "Ultra-High Resolution Interferometric Sensors", Optics and Photonics News, Michael Hercher, November, 1991, pp. 24-29, and involves the use of a detector array to detect the phase of a multi-phase optical signal.
Said patent includes means for generating an interference fringe pattern as a function of the displacement to be measured. It also includes a structured transducer, or detector, apparatus for simultaneously generating three intensity-modulated optical signals, I.sub.r. I.sub.s, and I.sub.t, which are related to the interference fringe pattern; electronic means for accumulating phase information proportional to the aspect of the interference fringe pattern; and means for converting the accumulated phase information to desired outputs indicative of the said displacement.
This technique, using a scale with 50 lines/mm., has demonstrated a resolution of less than a nanometer and an accuracy of .+-.10 nanometers. This performance is far in excess of that which can be achieved with conventional (Moire) encoder read-out techniques. Moire techniques generally allow interpolation of between one-eighth-scale spacing and one-sixteenth-scale spacing, corresponding to between one and three microns.
Devices based on this patent have been produced by Optra, Inc. for more than a year. Said devices, however, have used an array of discrete detector elements, each of which has been affixed to a base within a common plane. The outputs of these elements have then been routed off of the base to external, discrete electronic elements, which perform the required phase calculations.
This prior art has shown a number of shortcomings: first, it is expensive to manufacture. And secondly, it produces a relative phase measurement only, and is inaccurate for the generation of absolute phase information.
The current invention is an improved detector which generally performs the functions required for the above-cited patent in an improved way, with enhanced accuracy, and with lower cost and simplified manufacturing requirements.
The basis of the current invention is the incorporation of a multi-detector array onto a monolithic substrate, preferably a silicon PIN junction type.
The PIN technology is well-known, and is commonly used for photovoltaic detectors. PIN devices use an i-type semiconductor layer positioned between an n-type layer and a p-type layer. The following list contains several U.S. patents for PIN photovoltaic detectors.
______________________________________ U.S. Pat. No. Applicant Date Issued ______________________________________ 5,252,142 Matsuda et al. 10/12/93 5,246,506 Arya et al. 09/21/93 5,024,706 Tatsuyuki et al. 06/18/91 5,007,971 Tatsuyuki et al. 04/16/91 ______________________________________
In the preferred, three-phase version of this invention the first array element is electrically connected to the fourth element, the seventh element, etc., while the second element is connected with the fifth, eighth, etc., and the third is connected with the sixth, ninth, etc., forming three electrical channels. Thus, when the array is illuminated with an interference fringe pattern whose spacing is equal to approximately three times the detector spacing, three distinct electrical signals are formed, one in each such channel, wherein each signal is one-hundred-twenty degrees displaced in phase from each other signal.
In addition, a mask is used to limit the region of detection to the area of the major array elements, thus reducing spurious signals caused by photosensitive areas of the substrate outside of, or at the edges of, the detector array area.
One of the preferred implementations also contains the electronic amplifiers on the substrate itself, with provisions to accurately trim the gains of these amplifiers during the manufacturing process, so that each of the three channels has the same gain.
Although the three-phase implementation is the preferred implementation, the technique described in U.S. Pat. No. 5,098,190 works equally well with four-phase, five phase, etc. versions. As the number of phases increases, the ease of signal-to-phase conversion increases, at the expense of an increased number of signal channels.
FIGS. 1a through 1d illustrate this principle. FIG. 1a shows 60 elements electrically interconnected to form three electrical phases. FIG. 1c shows the same 60 elements interconnected to form five phases. FIGS. 1b and 1d show the time variations of the electrical signals of the various phases of each embodiment. A comparison of FIGS. 1b and 1c shows that, for the three-phase embodiment, there are three zero crossings within a period in which five zero crossings are produced by the five-phase implementation. Zero crossings are detected with a minimum of signal processing, as opposed to measurement of a non-zero signal level. Thus, if only zero crossings are detected, it is clear that the five-phase embodiment produces greater precision than the three-phase embodiment.
However, the higher the number of phases, the more expensive the detector becomes to manufacture. Each phase requires separate electrical amplifiers and conditioners. Furthermore, the electrical transfer function of each channel must be individually tuned during manufacturing to insure uniformity of the channels.
The three-phase implementation is therefore thought to be the optimum one, requiring the minimum in electronic components and manufacturing costs, while at the same time allowing a signal-to-noise-limited computation of phase.